Embedded control and monitoring functionalities have found their way into most electronic devices and the associated applications. However, this creates significant data flow that must somehow be connected to a central data gathering site. Thus, the need for connectivity of these various applications has given an explosive rise to connectivity solutions. In the early days of network connectivity, the network mesh utilized a wired connection, since these allow power and the reliable transmission of signals from a controller to its peripherals. However, in certain situations, it is difficult to physically dispose a peripheral in the controller and the wiring issues become more complex. To this end, wireless technology has seen a rise as the obvious solution although it has its own set of challenges, i.e., propagation, interference, security, regulations, and others.
One of the first solutions to the wireless issue was the rise of the IEEE Std. 802.11 Wireless Local Area Networks (WLAN). However, these WLANs are designed for high-end data networking. Among the system requirements of a WLAN are seamless roaming, messaging forwarding, longest possible range and capacity for a large population of devices. For some applications such as low data rate thermostats, etc., this would be overkill. Thus, other standards have come out for low data rate wireless personal area networks (LR-WPAN), which are designed for low-cost and very low-power short range wireless communications. These WPANs are designed to function in the Personal Operating Space (POS), extending up to ten meters in all directions and covering the area around a person whether stationary or in motion.
WPANs are utilized to convey information in the general vicinity of a user, which requires transmitting over relatively short distances. They typically consist of a central controller like device which is termed a Full Function Device (FFD) and peripherals which are referred to as Reduced Function Devices (RFD). The FFDs are powered and have the ability to relay packets, monitor multiple RFDs, etc. By comparison, the RFDs are very power efficient and have only the ability to communicate with the FFD.
In general, the IEEE came out with the 802.15 working group as defining three classes of WPANs. These are differentiated by data rate, battery drain and Quality of Service (QoS). IEEE Std. 802.15.3 is designated for high-data rate WPANs which can be utilized for multimedia applications. IEEE Std. 802.15.1 has been designated for use with medium-rate WPANs. These are designed for applications such as cable replacements for consumer electronic devices centered on mobile phones and Personal Digital Assistants (PDA) with a QoS suitable for voice applications. The last class of WPAN, the one that is the primary subject of this application, is the LR-WPAN class associated with the IEEE Std. 802.15.4. This is intended to serve applications enabled only by the low power end cost requirements not targeted to the other WPANs. These applications have a very low data rate and QoS that are typically not compatible with the higher data rate WPANs.
Typically, the IEEE 802 communication standards define a layered reference model that allows encapsulation of different levels of abstraction within a well defined functionality. 802.xx communication standards define only the bottom two layers of the International Standard Organization's (ISO's) Open System Connection (OSC) protocol reference model. These two layers are the physical (PHY) layer and the data link layer, which is the media access control (MAC) layer. Thereafter, additional layers are provided. These are the network layer and the application layer. These are referred to as the “upper layers.” To define these upper layers, various alliances have been formed that define the application for the particular 802.xx. communication standard. In the case of 802.15.4, one of these alliances is the ZigBee alliance. This is an organization that has developed a low power layer ISO/OSI reference model. These can be used for various things such as wireless sensors. Thus, ZigBee is a standards-based network protocol supported solely by the ZigBee alliance that uses the transport services of the IEEE 802.15.4 network specification. The ZigBee alliance is responsible for the ZigBee standard, and the IEEE is responsible for the physical transport specification. The ZigBee alliance provides the network protocol that rides on the transport specification, i.e., hence the layering concept.
The IEEE 802.15.4 standard defines multiple PHYs which span across three license-free frequency bands. One PHY spans the 868\9 15 MHz frequency band and the other PHY is dedicated to the 2.4 GHz frequency band, the one most commonly used and the one which will be described herein. The 2.4 GHz frequency band supports a total of 16 channels, channels 11 to 26. The data rate of the 2.4 GHz band allows a maximum data rate of 250 kbps. In general, the PHY layer is the interface to the physical radio and the generation of a radio link. The responsibilities of the PHY include receiver energy detection, link quality indication and clear channel assessment, in addition to transmitting and receiving packets across the electromagnetic medium. The ability to “Sniff” the air for other nodes is also an important aspect of the ZigBee specification.
The MAC layer is the layer that controls what is happening on the radio link. This provides control of access to the radio channel and employs the services of CSMA-CA (Carrier Sense Multiple Access-Collision Avoidance) to avoid collisions on the radio link. Network association and de-association are also duties that are handled by the MAC sublayer. Flow control, acknowledgement and retransmission of data packets, frame validation and network synchronization also falls in the domain of the MAC sub-layer. It is also the primary interface from the PHY to the upper application layers of the ZigBee application. It should be understood that ZigBee is just one application that utilizes the 802.15.4 transport standard and other applications could also be associated with that standard.
Low power RFDs have been developed to dispose the radio for the 2.4 GHz solution on a chip in the form of an offset-quadrature phase shift key (O-QPSK) modulation/demodulation scheme in conjunction with a PHY and a MAC. Typically, the application layer will be formed with the use of a second application layer chip. However, some single chip solutions have actually combined the PHY, MAC and at least a portion of the application layer onto a single chip to provide a single chip solution for a given alliance based application. The challenge to the designers of the single chip solution is to provide a low power radio, sufficient modulation/demodulation capability to handle the requirements of the 802.15.4 standard, in addition to the processing necessary to effectuate the MAC and application layers. In some applications, the MAC is constructed in hardware, as well as the PHY, with the application running mostly in software through the use of some type of microcontroller system.
The industry strives to develop single chip solutions for any type of application, if possible. One area that holds a great deal of promise is software-defined radio (SDR) concepts. The idea behind the SDR is to move the software as close to the antenna as possible. This can improve flexibility, adaptability and reduce the time-to-market. However, the ZigBee solution utilizing the 802.15.4 standard requires very low power operation. This low power operation can be at odds with the concept of a fully software-defined solution. The reason for this is that, as more functionality is moved over to the software side of the operation, the amount of processing power will increase. Processing power is directly correlated to power consumption, since the typical processing engine is a digital signal processor (DSP).
Another aspect associated with the ZigBee operation and its low power operation is the fact that it only operates a certain portion of the time. For example, the reduced function module (RFD) (the slave module) will only come on at certain times to “Sniff” the channel to determine if a transmission has occurred, which transmission is associated with a beacon signal. This requires the RFD to have fairly accurate timing information associated with it such that it can anticipate when a beacon signal will be transmitted with information. Thus, the RFD will turn on for a short period of time to Sniff the channel to determine if a packet is transmitted, process the packet if it exists and then turn off to wait for the next “window.” During this time, it is not possible to Sniff multiple packets, as only a single packet may be transmitted on that beacon for a particular RFD. Thus, it is important that the radio portion of the RFD be turned on, stabilized and perform a synchronization operation with a received packet and be able to obtain the following data which is termed a “payload.” Since there will not be another packet, after receipt of the packet, the radio will then turn off again. It must then have sufficient accuracy to only turn on when it anticipates another packet may be transmitted. It can be appreciated that the less time the radio is on and operating, the less power that will be consumed. If there is a 20% error in the time that the radio is required to be turned on due to timing considerations, this can significantly increase power. Thus, the need for accurate timing while the radio is off is important.